The present invention relates to a corrosion protection system for protecting a first and a second metal part of a marine construction, such as a marine surface vessel or a marine structure, the system comprising a first and a second anode and at least one reference electrode, the metal parts, the anodes, and the reference electrode being adapted to be at least partly immersed in an electrolyte in the form of water in which the marine construction is at least partly immersed. The invention also relates to a marine vessel with such a system, and also to a method in such a system. The method comprises controlling a first electrical current through a first electrical circuit, comprising the first anode, the first metal part and the electrolyte, at least partly based on a measured first electrical potential of the first metal part with at least one of the at least one reference electrode as a ground reference, and controlling a second electrical current through a second electrical circuit, comprising the second anode, the second metal part and the electrolyte, at least partly based on a measured second electrical potential of the second metal part with at least one of the at least one reference electrode as a ground reference.
In marine constructions, such as marine vessels and marine structures, a known way to protect an immersed metal part against galvanic corrosion is to provide a sacrificial anode, made of very pure zinc, magnesium, cast iron or an alloy of aluminum, which is directly fastened to, or electrically connected via a cable to the immersed metal part. In such a system, herein referred to as a passive corrosion protection system, or simply a passive system, the sacrificial anode will waste away, preventing damage to the immersed metal part.
An alternative to passive arrangements with sacrificial anodes is impressed current cathodic protection (ICCP) systems. In such a system, one or more active anodes are immersed and connected along with the immersed metal part to a common DC electrical power source. ICCP system active anodes usually have tubular or solid rod shapes, or are provided as continuous ribbons, and can include materials such as titanium, platinum, high silicon cast iron, graphite, mixed metal oxide, and niobium. Regardless whether the corrosion protection system is a passive system or of the ICCP type, a circuit is provided by an electrical current through the water serving as an electrolyte, and a surface polarization at the interface between the metal part and the water is created, serving to protect the metal part against corrosion.
In a passive system, as well as in an ICCP system, for a good corrosion protection, the electrical current between the anode and the metal part should be such that the surface polarization of the metal part is kept close to a desired value, or a desired interval, which depends on the material of the metal part. If said current is too low, the protection is too low, and the metal part will corrode at an undesired rate. For some metals, e.g. aluminum, damages can occur also if the current is too high. In general, too high of a current between the anode and the metal part gives calcium oxide precipitation which can stimulate excessive growth of sea weeds and sea animals on the metal part. Such growth is a particularly large concern in the case of recreational boats, which often stay docked giving opportunities for the growth.
ICCP systems usually include an electronic control unit (ECU) by means of which the electrical current between the anode and the metal part can be varied and controlled, which allows for control of the current between the anode and the metal part through the electrolyte, and thereby control of the surface polarization at an interface between the metal part and the electrolyte. Also passive systems can include such control, which can be obtained by an adjustable resistance in the electrical connection between the sacrificial anode and the metal part, for example as described in U.S. Pat. No. 5,627,414.
The control of the current is carried out based on measurements of a parameter indicative of said polarization. Such measurements are obtained by means of a reference electrode also immersed in the electrolyte. More specifically, the ECU is adapted to measure an electrical potential of the metal part with the reference electrode as a ground reference, and to control the current between the anode and the metal part based on these measurements and the desired value, or desired interval, of the electrical potential of the metal part with the reference electrode as a ground reference. This desired value, or desired interval, depends on a number of circumstances, for example, the type of reference electrode used, and the type of electrolyte. As an example, where the metal part is made of a material from a certain group of copper alloys, and the electrolyte is salt water, the desired value of the electrical potential of the metal part with a silver chloride coated silver reference electrode as a ground reference is suitably within the interval −450 mV-(−600) mV. As a further example, where the same type of reference electrode is used in the same type of electrolyte, and the metal part is made of stainless steel, the desired value of the electrical potential of the metal part with the reference electrode as a ground reference is suitably −800 mV.
A problem when measuring the electrical potential of the metal part with the reference electrode as a ground reference is that severe measurement errors, so called IR drops, can occur. The reason is that when a current runs between the anode to the metal part through the electrolyte, an electrical field will be generated in the electrolyte. This electrical field will affect said potential measurement, and might result in an off-set which gives a corresponding off-set at the control of the electrical current between the anode and the metal part. As a result, the corrosion protection will be less than optimal.
Further, if more than one system are provided in the marine construction for corrosion protection of respective metal parts, a current running through the electrolyte from the anode of one system to one of the metal parts, will generate an electrical field which will affect the measurement, with a reference electrode as a ground reference, of the electrical potential of a metal part protected by another system.
It should be noted that if more than one system are provided in the marine construction for corrosion protection of respective metal parts, the first and second metal parts have to be galvanically isolated. In this presentation, for practical recreational boats, galvanic isolation means that the electrical resistance between the first and second metal parts is sufficiently high, preferably at least 50ω, more preferably at least 500ω, and most preferably at least 10 kω.
To overcome said IR drop problem, it is known to locate the reference electrode in a vicinity of the metal part surface, so that the distance between them is in the order of a few millimeters. Disadvantages of this solution include the lack of flexibility in the arrangement of the corrosion protection system, since it places high demands on the placement and geometry of the reference electrode.
It is also known to provide a tube, one end of which is located very close to the metal part, and at the other end of which the reference electrode is provided, whereby the tube acts as a shielded compartment for electrolyte within it, so that the latter is not affected by the electrical field between the anode and the metal part. This solution also results in a lack of flexibility in the arrangement of the corrosion protection system, since it places high demands on the placement and geometry of said tube and the reference electrode.
It is desirable to improve the corrosion protection by corrosion protection systems in marine constructions.
It is also desirable to reduce errors in the measurement of the electrical potential of a metal part with a reference electrode as a ground reference in a corrosion protection system in a marine construction.
It is a further desirable to reduce errors in the measurement of the electrical potential of a metal part with a reference electrode as a ground reference in a corrosion protection system in a marine construction, while allowing flexibility in the arrangement of the reference electrode.
It is also desirable to reduce errors in the measurement of the electrical potential of a metal part with a reference electrode as a ground reference in a corrosion protection system in a marine construction, while allowing a compact arrangement of parts in said system.
A method according to an aspect of the present invention comprises repetitively performing the steps of                controlling the first and second electrical currents so as to be reduced or eliminated,        measuring at least one of the first and second electrical potentials while said electrical currents are maintained reduced or eliminated, and,        after measuring at least one of the first and second electrical potentials, controlling the first and second electrical currents so as to be increased or reestablished.        
Thus, during operation of the corrosion protection system, there are time periods of unreduced electrical currents between the respective anode and the respective metal part, and intermittent time periods of simultaneously reduced or eliminated (interrupted) electrical currents, during which the electrical potential of the metal part with the reference electrode as a ground reference is measured.
Since during the measurement of the electrical potential of the metal part with the reference electrode as a ground reference, there is no electrical current between the respective metal part and the respective anode, there is also no electrical field, in the electrolyte between the active anode and the metal part, which could disturb the measurement. Also, since during the measurement of the electrical potential of the first metal part with the reference electrode as a ground reference, there is no electrical current between the second metal part and the second anode, there is also no electrical field, in the electrolyte between the second anode and the second metal part, which could disturb the measurement of the electrical potential of the first metal part with the reference electrode as a ground reference, and vice versa. This will provide for very accurate measurements of the electrical potential of the metal part(s) with the reference electrode as a ground reference, which in turn will make the control of the electrical current between the respective anode and the respective metal part more accurate. This will of course improve the corrosion protection of the metal parts.
When the current between the respective anode and the respective metal part is reduced or switched off, the electrical field generated thereby in the electrolyte is reduced or disappears immediately. However, the polarization of the respective metal part, which can be viewed as resembling a capacitor, decreases relatively slowly, and for this reason, it will be possible to measure the electrical potential accurately shortly after the current is reduced or turned off, and thereafter increase or turn on the current again, without having lost any substantial amount of the polarization.
Also, the invention, according to an aspect thereof, does not put any restrictions on the placement of the reference electrode in relation to the respective anode and the respective metal part, which allows for a large degree of flexibility in the arrangement of the corrosion protection system. Advantageously, the invention, according to an aspect thereof, allows for including the anodes and the reference electrode in a common unit adapted to be mounted externally of the marine construction to be at least partially immersed in the electrolyte. Thereby, a compact arrangement of the corrosion protection system can be maintained while obtaining the improved accuracy of the electrical current control. As exemplified below, where the marine construction is a boat, the common unit could be an external unit mounted on the transom of a boat.
It should be noted that an aspect of the invention is particularly advantageous for recreational boats, which often stay docked and for this reason are sensitive to excessive underwater growth caused by over-protection, i.e. the current between the respective anode and the respective metal part being too high. In other words, for recreational boats, an accurate current control in the corrosion protection system is of particular importance.
Of course, the metal part can be any part needing corrosion protection, such as a boat hull, a boat propulsion unit (drive) with a propeller, a trim tab or a metal swimming ladder. Further, the invention is equally applicable to ships, regardless whether they are military, rescue, research, transportation ships, or of some other type, and to off-shore installations, such as oil drilling platforms.
It should be noted that one or more reference electrodes can be provided solely for the measurement of the electrical potential of each metal part. Alternatively, one or more reference electrodes, herein also referred to as a common reference electrode, can be shared by two or more metal parts.
Preferably, at least one of the at least one reference electrode is a common reference electrode adapted to be used for measuring both said electrical potentials. Thereby, the number of reference electrodes in the system can be kept low. It should be noted that the first and second metal parts should be galvanically isolated. In this presentation, for practical recreational boats, galvanic isolation means that the electrical resistance between the first and second metal parts is sufficiently high, preferably at least 50ω, more preferably at least 500ω, and most preferably at least 10 kω. The absence of galvanic isolation will result in equalization of the electrical potential difference between the electrical parts. This in turn will result in the first or the second anode having to “work harder”.
The common reference electrode adapted to be used for measuring both said electrical potentials can be integrated in the system in alternative manners. For example, measuring at least one of the first and second electrical potentials can comprise measuring the first potential while the second metal part is disconnected from the common reference electrode using a switch, and measuring the second potential while the first metal part is disconnected from the common reference electrode. Alternatively, both electrical potentials can be measured simultaneously, while it is secured that the electrical resistance between the first and second metal parts is sufficiently high, as mentioned above.
Preferably, the corrosion protection system comprises at least one impressed current cathodic protection system, at least the first anode being an active anode, and controlling the first electrical current comprises controlling an electrical power supply to the active anode and the first metal part.
Thereby, the invention, according to an aspect thereof, is adapted to an ICCP system. More specifically, controlling the first electrical current so as to be reduced or eliminated, comprises controlling the electrical power supply to the active anode and the first metal part so as to be reduced or discontinued. Further, measuring the first electrical potential while the first electrical current is maintained reduced or eliminated, comprises measuring the first electrical potential while the electrical power supply to the active anode and the first metal part is maintained reduced or discontinued. In addition, controlling said first electrical current so as to be increased or reestablished, comprises controlling the electrical power supply to the active anode and the first metal part so as to be increased or resumed.
Alternatively, or in addition, the corrosion protection system comprises at least one passive corrosion protection system, at least the second anode being a sacrificial anode, and controlling the second electrical current comprises controlling an adjustable resistance in an electrical connection between the sacrificial anode and the second metal part.
Thereby, the invention, according to an aspect thereof, is adapted to a passive corrosion protection system with a sacrificial anode, herein also referred to as a passive anode. More specifically, controlling the second electrical current so as to be reduced or eliminated, can comprise controlling the adjustable resistance so that the resistance thereof is increased, so that in turn the second electrical current is reduced or eliminated. Alternatively, controlling said second electrical current so as to be reduced or eliminated, can comprise controlling a relay or a switch so that the second current is eliminated. Further, measuring the second electrical potential while said second electrical current is maintained reduced or eliminated, comprises measuring the second electrical potential while the resistance of the adjustable resistance is maintained at an increased level, or, where applicable, while the second current is maintained eliminated by the relay or the switch. In addition, controlling the second electrical current so as to be increased or reestablished, comprises controlling the adjustable resistance so as to the resistance thereof is decreased, or, where applicable, controlling the relay or the switch so that the second current is reestablished.
Preferably, the steps of controlling the first and second electrical currents so as to be reduced or eliminated comprise eliminating or reducing the first and/or the second electrical currents at least 75%. Thereby, said electrical currents can be reduced from their respective momentary values in the current closed loop controls, i.e. the current values provided to obtain desired values for surface polarizations at the interfaces between the metal parts and the electrolyte, so as to reach a level being not more than 25% of said momentary values. In particularly preferred embodiments, the steps of controlling said electrical currents so as to be reduced or eliminated comprise eliminating or reducing said electrical currents at least 90%, preferably at least 95%. These reductions secure that the corresponding electrical fields in the electrolyte is reduced so as not to interference to any substantial degree with the measurement.
Where the invention is applied to an ICCP system, this can be done by discontinuing or reducing the electrical power to the active anode and the metal part so that the absolute value of an electrical tension applied between the active anode and the metal part is not more than 25% of the momentary absolute value of said electrical tension in the electrical power closed loop control.
However, it should be noted that in some embodiments, the first and/or the second electrical current is controlled so as to be eliminated while the electrical potentials of the metal parts with the reference electrode(s) as a ground reference are measured. Thereby, the first and/or the second electrical current are off completely during the measurement, so that there is no electrical field in the electrolyte between the respective anode and the respective metal part.
Preferably, as described closer below, the steps of controlling the first and second electrical currents so as to be reduced or eliminated and/or the steps of controlling the first and second electrical currents so as to be increased or reestablished comprise controlling the first and/or the second electrical current according to a ramp function.
Preferably, each time period, during which said electrical currents are maintained reduced or eliminated, is not above 1 second, more preferably not above 500 ms. Preferably, a ratio between time periods, during which said electrical currents are not maintained reduced or eliminated, and time periods, during which said electrical currents are maintained reduced or eliminated, is at least ten. As explained closer below, this will secure that enough time is provided for maintaining the polarization of the metal part in relation to the intermittent periods for measurement of the electrical potential of the metal part.